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Nitya Murthy, Adam Hickenbotham; Maximum Visual Acuities are Located Outside the Circle of Least Confusion. Invest. Ophthalmol. Vis. Sci. 2018;59(9):4765.
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© ARVO (1962-2015); The Authors (2016-present)
Current methods of subjective refraction assume that when a maximum visual acuity is achieved, the Circle of Least Confusion (CoLC) has been centered on the outer limiting membrane of the retina. This study aims to evaluate whether better acuity occurs when an astigmatic focus is placed on the retina rather than the CoLC. The Jackson Cross Cylinder (JCC) is a spherocylindrical lens with a Spherical Equivalent (SE) of zero. It is designed to maintain the placement of the CoLC on the retina while providing a patient with two alternative choices of identical SE. We tested the hypothesis that individuals will often not prefer to place the CoLC on the retina as better vision can be achieved when it is placed at a different location.
Subjects were students (n=54, 36 females, 18 males, mean age=24.2±2.2 years old) at the Kentucky College of Optometry participating in an optics laboratory. Each subject’s spectacle correction and BCVA were measured using standard subjective refraction techniques. A ±0.25 D JCC was introduced and VA was re-measured. A spherical over-refraction (SOR) was then performed, recording the minimum spherical power added to achieve maximum VA. This technique was repeated with ±0.50 D JCC and ±1.00 D JCC.
The mean SOR measured while viewing through a ±0.25 D, ±0.50 D, and ±1.00 D JCC were -0.34(±0.40), -0.53(±0.48), and -0.65(±0.67) respectively. Visual acuities improved from 0.00(±0.10), 0.15(±0.12), and 0.37(±0.13) LogMAR at the CoLC to -0.05(±0.08), 0.07(±0.12) and 0.26(±0.17) LogMAR following SOR. Each of these results were significant with P-values of 1.6E-05, 1.8E-07, and 3.7E-06 when compared to the null hypothesis that subjects obtain their best visual acuities with the CoLC on the retina.
The Circle of Least Confusion did not provide best visual acuity. Maximum visual acuity was most frequently achieved when the CoLC was placed behind the retina in a state of hyperopic astigmatism. 72% of subjects preferred that the CoLC was placed behind the retina compared to 16% who preferred it on the retina and 12% who preferred it in front of the retina. These results have significant impact on the theory and techniques used for manifest subject refraction for spectacle correction, contact lens fitting, and refractive surgery.
This is an abstract that was submitted for the 2018 ARVO Annual Meeting, held in Honolulu, Hawaii, April 29 - May 3, 2018.
Graph showing number of subjects with a measured SOR (y-axis) vs measured SOR in diopters (x-axis) demonstrating hyperopic shift in preferred correction
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